Power Device With Trenches Having Wider Upper Portion Than Lower Portion

ABSTRACT

A method of forming a semiconductor device includes the following. A masking layer with opening is formed over a silicon layer. The silicon layer is isotropically etched through the masking layer openings so as to remove bowl-shaped portions of the silicon layer, each of which includes a middle portion and outer portions extending directly underneath the masking layer. The outer portions form outer sections of corresponding trenches. Additional portions of the silicon layer are removed through the masking layer openings so as to form a middle section of the trenches which extends deeper into the silicon layer than the outer sections of the trenches. A first doped region of a first conductivity type is formed in an upper portion of the silicon layer. An insulating layer is formed within each trench, and extends directly over a portion of the first doped region adjacent each trench sidewall. Silicon is removed from adjacent each trench until, of the first doped region, only the portions adjacent the trench sidewalls remain. The remaining portions of the first doped region adjacent the trench sidewalls form source regions which are self-aligned to the trenches.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/049,996, filed Mar. 17, 2008, which is a continuation of U.S.application Ser. No. 11/111,305, filed Apr. 20, 2005, which is adivision of U.S. application Ser. No. 10/442,670, filed May 20, 2003,now U.S. Pat. No. 6,916,745, all of which are incorporated herein byreference in their entirety for all purposes.

BACKGROUND

The present invention relates to semiconductor MOSFET technology andmore particularly to a trench MOSFET having self-aligned features.

Power MOSFETs (metal oxide semiconductor field effect transistors) arewell known in the semiconductor industry. One variety of power MOSFETsis the vertically-conducting trench MOSFET. A cross-section view of sucha MOSFET is shown in FIG. 1. MOSFET 100 has trenches 111 each includinga polysilicon gate 112 insulated from body regions 114 by a gatedielectric 110. Source regions 116 flank each side of trenches 111.Dielectric layer 120 insulates gates 112 from overlying metal layer 126.Substrate region 102 forms the drain of MOSFET 100.

When MOSFET 100 is biased in the on state, current flows verticallybetween source regions 116 and substrate 102. The current capability ofMOSFET 100 in the on state is a function of the drain to sourceresistance (Rds_(on)). To improve the current capability of the MOSFET,it is necessary to reduce the Rds_(on). One way to reduce the Rds_(on)of the trench MOSFET is to increase the trench density (i.e., toincrease the number of trenches per unit area). This may be achieved byreducing the cell pitch. However, reducing the cell pitch of MOSFETs islimited by the particulars of the MOSFET cell structure and the specificprocess recipe used to manufacture the MOSFET. Reducing the cell pitchis made further difficult by such limitations of the manufacturingprocess technology as the minimum critical dimensions thephotolithography tools are configured to resolve, the minimum requiredspacing between different cell regions as dictated by the design rules,and the misalignment tolerances.

The different dimensions that determine the minimum cell pitch fortrench MOSFET 100 are shown in FIG. 1. Dimension A is the minimum trenchwidth the photolithography tools are configured to resolve, dimension Bis the minimum contact opening the photolithography tools are configuredto resolve, dimension C is the minimum trench-to-contact spacingdictated by the design rules, and dimension D is the contactregistration error tolerance or contact misalignment tolerance. Theminimum cell pitch for MOSFET 100 thus equals A+B+2C+2D. Reduction ofany of these dimensions without complicating the process technology isdifficult to achieve.

Thus, a new approach wherein the cell pitch of the trench MOSFET can bereduced without increasing the process complexity is desirable.

SUMMARY

In accordance with an embodiment of the present invention, asemiconductor device is formed as follows. An exposed surface area of asilicon layer where silicon can be removed is defined. A portion of thesilicon layer is removed to form a middle section of a trench extendinginto the silicon layer from the exposed surface area of the siliconlayer. Additional exposed surface areas of the silicon layer wheresilicon can be removed are defined. Additional portions of the siliconlayer are removed to form outer sections of the trench such that theouter sections of the trench extend into the silicon layer from theadditional exposed surface areas of the silicon layer. The middlesection of the trench extends deeper into the silicon layer than theouter sections of the trench.

In another embodiment, a gate electrode partially filling the trench tobelow the outer sections of the trench is formed.

In another embodiment, the silicon layer comprises a body region.Impurities are implanted to form a first region in the body region. Thefirst region extends along a surface of the body region and directlybelow the outer sections of the trench.

In another embodiment, a dielectric layer extending only across a topsurface of the trench is formed. Exposed silicon is removed until: (i)of the first region, only portions located substantially directly belowthe outer sections of the trench remain, the remaining portions of thefirst region forming source regions of the semiconductor device, and(ii) a surface area of the body region becomes exposed.

In another embodiment, a sidewall of the dielectric layer is exposed.The exposed sidewall of the dielectric layer together with an exposedsidewall of each source region forms a sidewall of a contact opening forcontacting the body region and source region.

In accordance with another embodiment of the present invention, asemiconductor device is formed as follows. A masking layer is formedover a silicon layer. The masking layer has an opening through which asurface area of the silicon layer is exposed. The silicon layer isisotropically etched through the masking layer opening so as to remove abowl-shaped portion of the silicon layer. The bowl-shaped portion has amiddle portion along the exposed surface area of the silicon layer andouter portions extending directly underneath the masking layer. Theouter portions of the removed silicon layer which extend directlyunderneath the masking layer form outer sections of a trench. Additionalportions of the silicon layer are removed through the masking layeropening so as to form a middle section of the trench which extendsdeeper into the silicon layer than the outer sections of the trench.

In accordance with yet another embodiment of the present invention, asemiconductor device is formed as follows. A plurality of trenches isformed in a silicon layer. A first region of a first conductivity typeis formed in the silicon layer. An insulating layer filling an upperportion of each trench is formed. Exposed silicon is removed until atleast: (i) an edge of the insulating layer in each trench is exposed,and (ii) of the first region, only a portion adjacent each trenchsidewall remains. The remaining portion of the first region adjacenteach trench sidewall forms a source region of the semiconductor device.

In accordance with another embodiment of the present invention, asemiconductor device comprises a trench in a silicon layer. A sourceregion is in the silicon layer adjacent each sidewall of the trench. Thetrench sidewalls are shaped along the silicon layer such that the trenchsidewalls fan out near the top of the trench to extend directly over atleast a portion of each source region.

In another embodiment, a gate electrode partially fills the trench butoverlaps each source region along the trench sidewalls. An insulatinglayer substantially fills a remaining portion of the trench over thegate electrode. A sidewall of the insulating layer in the trenchtogether with a sidewall of a corresponding source region form asidewall of a contact opening through which contact is made at leastwith the source region.

In another embodiment, a body region is adjacent each trench sidewall,and the body regions are of opposite conductivity type to that of thesource regions. A metal layer contacts the body regions and the sourceregions through the contact opening.

In accordance with another embodiment of the present invention, asemiconductor device comprises a plurality of trenches in a siliconlayer. An insulating layer fills an upper portion of each trench. Asource region is in the silicon layer adjacent each trench sidewall suchthat a sidewall of each insulating layer together with a sidewall of acorresponding source region forms a contact opening between every twoadjacent trenches.

The following detailed description and the accompanying drawings providea better understanding of the nature and advantages of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section view of a conventional trench MOSFET;

FIGS. 2A-2K show cross-section views at different stages ofmanufacturing a trench MOSFET in accordance with an embodiment of thepresent invention;

FIG. 3 is a graph showing the effect of cell pitch reduction onRds_(on);

FIGS. 4A and 4B show an alternate method for forming trenches inaccordance with another embodiment of the invention; and

FIG. 5 is an exemplary cross-section view corresponding to that in FIG.2K, and is provided to show a more accurate representation of thecontours of the trenches in accordance with one embodiment of theinvention.

DETAILED DESCRIPTION

In accordance with the present invention, a structure and method forforming a trench MOSFET having self-aligned features which result incell pitch reduction without increasing the process complexity aredisclosed. In one embodiment, trenches are formed in an epitaxial layerin such manner that the trench sidewalls fan out near the top of thetrench over source regions. An insulating layer formed along a topportion of each trench together with the source regions defines thecontact openings between adjacent trenches for contacting the source andbody regions. This structure and method of forming the trenches leads toa MOSFET which has source regions and contact openings self-aligned tothe trenches. This in turn enables the 2D portion of the cell pitch ofprior art MOSFET 100 (FIG. 1) to be eliminated and the dimension B to bereduced to thus obtain a reduced cell pitch without introducing anyprocess complexities

FIGS. 2A-2K are cross-section views at different stages of manufacturinga trench MOSFET in accordance with an embodiment of the presentinvention. In FIG. 2A, a lightly doped N-type epitaxial layer 204extends over a highly-doped N-type substrate 202. A layer of a materialwhich is resistant to silicon etch having a thickness in the range of2,000-10,000 Å is formed over epitaxial layer 204. In one embodiment, anoxide layer having a thickness of about 5,000 Å is used. Using a maskingstep, predefined portions of the layer of material resistant to siliconetch are removed so that only regions 206 remain. In the embodimentwherein an oxide layer is used, conventional dry or wet etch may be usedto remove the predefined portions of the oxide layer.

In FIG. 2B, a first silicon etch is carried out to form a mid-section208 of a plurality of trenches. The spacing between regions 206 definesthe width of mid-section 208 which is in the range of 0.2-2.0 μm.Mid-section 208 extends from the exposed surface areas of epitaxiallayer 204 to a depth in the range of 0.5-3.0 μm. In one embodiment, thewidth and depth of mid-section 208 are about 0.35 μm and 1.0 μm,respectively. Conventional methods for etching silicon, for example,reactive ion etching (REI), may be used to form mid-section 208 of thetrenches.

In FIG. 2C, portions of regions 206 are removed to expose additionalsurface areas 207 of epitaxial layer 204. Smaller regions 206a having athickness in the range of 1,000-9,000 Å thus remain. In the embodimentwhere regions 206 are from oxide, regions 206 are isotropically etchedso that smaller oxide regions 206 a having a thickness of about 2,500 Åremain.

In FIG. 2D, a second silicon etch is carried out to remove portions ofepitaxial layer 204 along its exposed surfaces to thereby form outersections 208 b of the trenches. As shown, mid-section 208 a extendsdeeper than outer sections 208 b. Outer sections 208 b extend fromsurface areas 208 b of epitaxial layer 204 to a depth in the range of0.1-1.0 μm. In one embodiment, the depth of outer sections 208 b isabout 0.4 μm. Note that the second silicon etch also removes siliconfrom along the bottom of the mid-section 208 though it is not necessaryto do so. As with the first silicon etch, conventional methods foretching silicon, for example, reactive ion etching (REI), may be usedfor the second silicon etch.

While FIGS. 2A-2D show one method for forming trenches having a deepmid-section and shallow outer sections, the invention is not limited tothis particular method. For example, an alternate method for formingtrenches having similar physical characteristics is shown in FIGS. 4Aand 4B. After forming isolated regions 206 of for example oxide orphotoresist, as in FIG. 2A, an isotropic silicon etch is carried out sothat openings 203 are created in epitaxial layer 204 between adjacentregions 206 as shown in FIG. 4A. The isotropic etch removes silicon fromunder regions 206 as shown. Next, keeping regions 206 intact, aconventional silicon etch is carried out to form deeper mid-sections 203a of the trenches as shown in FIG. 4B. As can be seen, each trench has adeep mid-section 203 a and shallow outer sections 203 b extending underregions 206.

Referring back to FIGS. 2A-2K, in FIG. 2E, remaining regions 206 a mayoptionally be removed at this stage of the process. An insulating layer210 is then formed along the surface of epitaxial layer 204 usingconventional methods. Sidewalls of the trenches are thus coated withinsulating layer 210. Insulating layer 210 has a thickness in the rangeof 50-1,000 Å. In one embodiment, insulating layer 210 is a gate oxidehaving a thickness of about 400 Å.

Next, using conventional polysilicon deposition techniques, apolysilicon layer 212 having a thickness in the range of 1,000-15,000 Åis deposited over insulating layer 210 to fill the trenches. In oneembodiment, polysilicon layer 212 has a thickness of about 5,500 Å andis doped with impurities. In yet another embodiment, prior to formingpolysilicon layer 212, a thick insulating layer is formed along thebottom of the mid-section 208 a of the trenches. This advantageouslyreduces the gate capacitance of the MOSFET.

In FIG. 2F, polysilicon layer 212 is etched back to form gates 212 a inmid-section 208 a of the trenches. Polysilicon layer 212 is etched backsuch that its upper surface is recessed below the outer sections 208 bof the trenches. This insures that no polysilicon is left in the outersections 208 b of the trenches which may otherwise short the gate to thesource and also block the source and body implants carried out later inthe process. However, the extent to which the polysilicon layer 212 isetched back must be carefully controlled to insure that at least aportion of the gate overlaps with the source regions formed in latersteps. Conventional polysilicon etching techniques may be used to etchback polysilicon layer 212.

P-type body regions 214 are then formed in epitaxial layer 204 betweenadjacent trenches by implanting P-type impurities such as boron. TheP-type implant is symbolically shown by arrows 218 which indicate thatno mask is needed. Body regions 214 extend into epitaxial layer 204 to adepth primarily dictated by the target channel length. Next,highly-doped N-type regions 216 are formed in body regions 214 byimplanting N-type impurities such as arsenic or phosphorous. N-typeregions 216 extend along the top surface of body regions 214 anddirectly below outer sections 208 b of the trenches. The N-type implantis symbolically shown by arrows 219 which indicate that no masking isneeded for this implant either. Conventional ion implantation techniquesmay be used for both implant steps.

In FIG. 2G, a dielectric layer 220, such as BPSG, is formed over theentire structure using conventional techniques. Dielectric layer 220 hasa thickness in the range of 2,000-15,000 Å. In one embodiment, thethickness of dielectric layer 220 is about 8,000 Å. Next, a conventionaldielectric flow step is carried out to obtain a planar surface as shownin FIG. 2H. dielectric layer 220 a is then etched until silicon isreached as shown in FIG. 2I. After the dielectric etch, dielectricregions 220 b which are fully contained in the trenches remain whilesurface areas of N-type regions 216 are exposed.

In FIG. 2J, a conventional silicon etch is carried out to form contactopenings 222. Sufficient amount of silicon is removed so that along withthe upper portion of N-type regions 216 a top layer of body regions 214is also removed. This insures that: (i) a top surface of body regions214 a becomes exposed so that contact can be made to body regions 214 a,(ii) of N-type region 216, source regions 216 a separated by bodyregions 214 a remain, and (iii) sidewall areas of source regions 216 abecome exposed so that contact can be made to source regions 216 a. InFIG. 2K, metal layer 226 is deposited to contact body regions 214 a andsource regions 216 a. Before metal 226 is deposited, a layer of heavilydoped P-type region 224 may optionally be formed along the top surfaceof body regions 214 a using conventional ion implantation techniques.The heavily doped region 224 helps achieve an ohmic contact betweenmetal 226 and body region 214 a. As shown, metal layer 224 is insulatedfrom gates 212 a by the dielectric layer 220 b extending along the topsurface of each trench.

Referring back to FIG. 2J, the silicon etch carried out to form contactopenings 222 exposes portions of insulating layer 210 extending alongthe sidewalls of outer sections 208 b of the trenches. As can be seen,the exposed portions of insulating layer 210 together with the exposedsidewall area of source regions 216 a advantageously define contactopenings 222 between adjacent trenches. Thus, with no masking steps usedin forming either source regions 216 a or contact openings 222, sourceregions 216 a and contact openings 222 which are self-aligned to thetrenches are formed.

Because source regions 216 a and contact openings 222 are self-alignedto the trenches, the need to account for contact misalignment as inconventional techniques (dimension D in FIG. 1) is eliminated.Furthermore, the contact openings (dimension B in FIG. 1) can be madesmaller than the photolithography tools are typically configured toresolve. Thus, not only the 2D term is eliminated from the minimum cellpitch A+B+2C+2D of the conventional trench MOSFET in FIG. 1, but theterm B can be made much smaller. For the same process technology, a muchsmaller cell pitch is therefore obtained without increasing the processcomplexity.

The small cell pitch results in an increase in the number of trenchesper unit area which in turn has the desirable effect of lowering theRds_(on). This is more clearly shown in FIG. 3. FIG. 3 is a graphshowing the effect of cell pitch reduction on Rds_(on). The verticalaxis represents Rds_(on) and the horizontal axis represents the cellpitch. The numbers along the vertical axis are merely illustrative anddo not reflect actual values of Rds_(on). Two curves are shown with theupper curve corresponding to a gate-source bias of 4.5V and the lowercurve corresponding to a gate-source bias of 10V. For the same processtechnology, the self-aligned features of the present invention result ina reduction of the cell pitch from 1.8 μm to 1.0 μm. This cell pitchreduction results in about a 30% reduction in Rds_(on) in the case of10V biasing and about a 25% reduction in the case of 4.5V biasing.

The cross-section views in FIGS. 2A-2K are merely illustrative and arenot intended to limit the layout or other structural aspects of the cellarray. Furthermore, these figures may not accurately reflect the actualshape of all the various regions as they would appear in an actualdevice. FIG. 5 is an exemplary cross-section view corresponding to thatin FIG. 2K, and is provided to show a more accurate representation ofthe contours of the trenches in accordance with one embodiment of theinvention. Because of the small dimensions of some of the regions andthe effects of such processing steps as temperature cycles, a roundingof many of the corners occurs during processing. As a result, thetrenches appear Y-shaped as shown in FIG. 5 rather than T-shaped asshown in FIG. 2K. However, it is to be understood that the invention isnot limited to a particular shape of the trenches.

While the above is a complete description of the embodiments of thepresent invention, it is possible to use various alternatives,modifications and equivalents. For example, the process steps depictedin FIGS. 2A-2K are for manufacturing an N-channel MOSFET. Modifyingthese process steps to obtain an equivalent P-channel MOSFET would beobvious to one skilled in the art in light of the above teachings.Similarly, modifying the process steps to obtain other types ofsemiconductor devices such as insulated gate bipolar transistor (IGBT)would be obvious to one skilled in the art in light of the aboveteachings.

Also, body region 214 (FIG. 2F) may be formed earlier in the processingsequence. For example, in FIG. 2A, prior to forming regions 206, P-typeimpurities may be implanted into epitaxial layer 204 or a P-typeepitaxial layer may be grown over epitaxial layer 204. Similarly, N-typeregions 216 (FIG. 2F) may be formed earlier in the processing sequence.For example, a blanket implant of N-type impurities may be carried outto form a highly-doped N-type region in the body region before formingthe trenches. The highly-doped N-type region however needs to extenddeeper into the body region than that depicted in FIG. 2F so that afterthe trenches are formed, at least a portion of the N-type region extendsbelow the outer sections of the trenches. Also, a deeper silicon etchwould be required in FIG. 2J in order to reach a surface of the bodyregion.

In a further variation, epitaxial layer 204 may have a graded dopingconcentration rather than a fixed doping concentration, or may be madeof a number of epitaxial layers each having a different dopingconcentration, or may be eliminated all together depending on the designgoals. Moreover, the trenches may extend clear through epitaxial layer204 and terminate within substrate 202.

Therefore, the scope of the present invention should be determined notwith reference to the above description but should, instead, bedetermined with reference to the appended claim, along with their fullscope of equivalents.

1. A method of forming a semiconductor device, comprising: forming amasking layer over a silicon layer, the masking layer having openingsthrough which surface areas of the silicon layer are exposed;isotropically etching the silicon layer through the masking layeropenings so as to remove bowl-shaped portions of the silicon layer, eachbowl-shaped portion having a middle portion and outer portions extendingdirectly underneath the masking layer, the outer portions forming outersections of corresponding trenches; removing additional portions of thesilicon layer through the masking layer openings so as to form a middlesection of the trenches which extends deeper into the silicon layer thanthe outer sections of the trenches; forming a first doped region of afirst conductivity type in an upper portion of the silicon layer;forming an insulating layer within each trench, the insulating layer ineach trench extending directly over a portion of the first doped regionadjacent each trench sidewall; and removing silicon from adjacent eachtrench until, of the first doped region, only the portions adjacent thetrench sidewalls remain, the remaining portions of the first dopedregion adjacent the trench sidewalls forming source regions which areself-aligned to the trenches.
 2. The method of claim 1 wherein a topsurface of the insulating layer is substantially coplanar with a topsurface of the first doped region.
 3. The method of claim 1 wherein thestep of removing exposed silicon forms a contact opening between everytwo adjacent trenches.
 4. The method of claim 3 further comprising:prior to forming the first doped region, forming a second doped regionof a second conductivity type in the upper portion of the silicon layer.5. The method of claim 4 further comprising: implanting dopants throughthe contact openings to form a heavy body region of a secondconductivity type in the second doped region.
 6. The method of claim 5further comprising: forming a metal layer to electrically contact thesource regions and the heavy body region through the contact opening. 7.The method of claim 1 further comprising: prior to forming theinsulating layer, forming a gate electrode partially filling eachtrench.
 8. A method of fabricating a semiconductor device, comprising:forming a masking layer over a silicon layer, the masking layer havingopenings through which surface areas of the silicon layer are exposed;isotropically etching the silicon layer through the masking layeropenings so as to remove bowl-shaped portions of the silicon layer, eachbowl-shaped portion having a middle portion and outer portions extendingdirectly underneath the masking layer, the outer portions forming outersections of corresponding trenches; removing additional portions of thesilicon layer through the masking layer openings so as to form a middlesection of the trenches which extends deeper into the silicon layer thanthe outer sections of the trenches; forming a gate electrode partiallyfilling each trench; forming a first doped region of a firstconductivity type in an upper portion of the silicon layer; forming aninsulating layer within each trench such that a top surface of theinsulating layer is substantially coplanar with a top surface of thefirst doped region, each trench having a contour such that theinsulating layer in each trench extends directly over a portion of thefirst doped region adjacent each trench sidewall; and removing exposedsilicon from between adjacent trenches such that: 1) a contact openingis formed between every two adjacent trenches, and 2) of the first dopedregion, only the portion adjacent each trench sidewall remains, theportion of the first doped region remaining adjacent each trenchsidewall forming a source region.
 9. The method of claim 8 wherein thestep of removing exposed silicon is carried out without using a mask sothat the contact opening between every two adjacent trenches and theportion of the first doped region remaining adjacent each trenchsidewall are self-aligned.
 10. The method of claim 8 further comprising:prior to forming the first doped region, forming a second doped regionof a second conductivity type in the upper portion of the silicon layer.11. The method of claim 10 further comprising: implanting dopantsthrough the contact openings to form a heavy body region of a secondconductivity type in the second doped region between adjacent trenches.12. The method of claim 11 further comprising: forming a metal layer toelectrically contact the source regions and the heavy body regionthrough the contact opening.